Experimental study on mechanical properties and durability of basalt fiber reinforced coral aggregate concrete

Construction and Building Materials 237 (2020) 117628

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Experimental study on mechanical properties and durability of basalt fiber reinforced coral aggregate concrete Ditao Niu a,b, Li Su b,⇑, Yang Luo b, Daguan Huang b, Daming Luo a,b a b

State Key Laboratory of Green Building in Western China, Xi’an University of Architecture and Technology, Xi’an, China Department of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an, China

h i g h l i g h t s
The effects of BF on the mechanical properties of CAC at different curing ages are studied.
Detailed chloride contents brought by coral aggregates of BFRCAC are recorded, including total and free chloride content.
Water absorption and sorptivity properties of BFRCAC are investigated.
TG-DTG and FE-SEM are used to study microstructure of BFRCAC.

a r t i c l e

i n f o

Article history: Received 15 July 2019 Received in revised form 25 October 2019 Accepted 15 November 2019

Keywords: Basalt fiber Reinforced coral aggregate concrete Mechanical properties Chloride content carried in coral aggregate Sorptivity Microstructure

a b s t r a c t With the development of marine technology, the potential of coral aggregate concrete (CAC) for use in the construction of reefs has been identified. However, the shape, surface structure, high porosity, and chloride-ion-carrying properties of coral aggregates yield special microstructures and affect the mechanical properties and durability of CAC. This study investigates the effect of basalt fiber (BF) on the mechanical properties, chloride content carried in coral aggregates, and water absorption of CAC. The results indicate that with the increase in fiber content, the mechanical properties and water absorption resistance of basalt fiber reinforced coral aggregate concrete present an increasing trend then followed decreasing trend. Furthermore, 0.05% BF yields the highest improving effect on the mechanical properties and water absorption resistance at 28 days. Compared with the reference concrete, the compressive strength and splitting tensile strength of CAC with 0.05% BF increase by 9.87% and 1.36% at 28 days, respectively. When the fiber content continues to increase, the strengthening effect decreases, and cause adverse effects. The dissolution of chloride in coral aggregates was accelerated by 0.05% BF, while the chloride dissolution was inhibited by 0.1%–0.2% BF. Moreover, the microstructure of CAC is investigated to elucidate the enhancement mechanism of BF in terms of thermogravimetric analysis, theoretical total pore volume, fiber bonding performance, and fiber-matrix interfacial characteristics. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction Rich marine resources are important for human sustainable development [1]. With the rapid development of emerging marine industries, breakthroughs have been made in the construction of marine infrastructure. Marine concrete provides the foundation for supporting the development of marine engineering. Most islands are far from the mainland. The transport of aggregates from the mainland to reefs increases costs; additionally, it is also affected by severe climate (such as typhoons) and the construction period cannot be guaranteed. Without destroying the natural eco⇑ Corresponding author. E-mail address: [email protected] (L. Su). https://doi.org/10.1016/j.conbuildmat.2019.117628 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

logical environment of reefs, the use of coral reefs produced by dredging channels to prepare concrete can solve the shortage of materials and reduce the environmental load on the islands [2,3]. It has been reported [4–7] that using coral reefs to prepare concrete is feasible, and that the strength can satisfy engineering requirements. However, the durability caused by chloride salt must be considered. Compared with ordinary limestone and basalt aggregates, coral reefs exhibit distinct characteristics of irregular shape, rough surface, and high porosity [8]. These characteristics necessitate a large amount of cement and water to prepare coral aggregate concrete (CAC) to satisfy the working performance requirements [9]. The apparent density and strength of hardened CAC are lower than those of ordinary concrete. Chloride attack is a severe problem in the application of CAC structure. Hence, chlo-

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D. Niu et al. / Construction and Building Materials 237 (2020) 117628

ride ion erosion permeating in coral aggregates must be investigated. Poor performances limit the application and development of CAC in reef engineering. The addition of fibers to concrete can effectively prevent cracks from occurring and expanding, reduce stress concentration at cracks, create a more continuous and uniform stress field in concrete, and increase the toughness [10]. Studies have shown that adding fibers to concrete can effectively improve the mechanical properties and durability of concrete, thereby obtaining high strength, high toughness, and high durability in fiber reinforced concrete [11,12]. According to the elastic modulus, fibers are classified as flexible fibers (polypropylene fibers and nylon fibers) or stiff fibers (steel fibers, carbon fibers, and basalt fibers). Flexible fibers can improve the crack resistance and impact resistance of concrete owing to their high ductility, while stiff fibers can improve the strength of concrete to some extent. Basalt fiber (BF) is made from natural volcanic basalt rock by melting and drawing. It possesses excellent physical and mechanical properties, including high temperature stability, good acid alkali resistance, high tensile strength, and excellent plastic deformation capacity. Further, it is a cost-effective environmentally friendly inorganic fiber [13–15]. BF has recently gained popularity in concrete reinforcement applications owing to its excellent mechanical properties and environmentally friendly manufacturing processes. Ramachandran et al. studied the chemical durability of BF as early as 1981. Their results demonstrated the potential application of BF in reinforced cement [16]. Branston et al. [13] evaluated the effects of two types of BF (bundle dispersion fiber and minibar) on the mechanical properties of concrete. The results indicated that both types of fibers improved the pre-crack strength. Ye et al. [17] reported that BFs possess good alkali resistance and can significantly improve the toughness and crack resistance of concrete. Dias et al. [18] studied the effect of BFs of different volume fractions on the fracture toughness of concrete beams. Their experimental results indicated that the ultimate load and deformation of BF concrete beams increased significantly before failure, whereas the sensitivity to cracks decreased. Zielinski and Olszewski [19] investigated the physical and mechanical properties of cement mortar reinforced with BF at 28 days and obtained the optimum mixing amount of BF. Wang et al. [20,21] experimentally studied the mechanical properties of CAC with sisal and polypropylene fibers in different contents; their results indicated that the fibers could improve the brittleness of CAC, thereby affording good ductility, increased compressive strength, splitting tensile strength, flexural strength, and elastic modulus, as well as changing the failure mode. However, the improvement varies across a range, with the improvement in compressive strength being limited, and the effect on the flexural strength and elastic modulus of CAC being more significant. When the fiber content exceeds the optimal amount, the strength tends to decrease [22]. A review of the aforementioned studies reveals that studies regarding the mechanical properties and durability of BF reinforced concrete are insufficient. Furthermore, studies on basalt fiber reinforced coral aggregate concrete (BFRCAC) are scarce, which hinders the further application and development of BFRCAC. The main objectives of this study are to investigate the mechanical properties, chloride content permeating in coral aggregates and sorptivity of BFRCAC and to determine the optimal content of BF to improve concrete properties considerably. In addition, through thermogravimetric (TG) analysis and field emission scanning electron microscope (FE-SEM), the microstructure of BFRCAC, such as porosity, and fiber bonding performance, as well as the fiber  matrix interfacial transition zone (ITZ) were examined in detail.

2. Materials and test methods 2.1. Materials To prepare the BFRCAC specimens, P.O 42.5 Portland cement (OPC), provided by Xi’an Qinling Cement Co., Ltd. and fly ash (FA), produced by Datangshenglong Technology Industry Co., Ltd. were used as cementitious materials. The chemical composition and physical properties of the binder are listed in Tables 1–3. A polycarboxylic-based superplasticiser (PBS) was prepared by Jiangsu Bote New Materials Co., Ltd., with water reducing rate of 30%, solid content of 40%, and alkali content of 6.5%. The local drinkable tap water (W) in Xi’an was used for mixing. The BF is shown in Fig. 1 and the physical and mechanical properties are presented in Table 4. Coral reefs, dredged from coral reef channel in the South China Sea, were provided by a company in Xiamen, China, as shown in Fig. 2. The original mined coral reefs contain pungent seawater and salt. Original coral reefs exhibit different shapes and sizes, for example, deer horn, flake, rod-like, and honeycomb-like, which lead to the poor gradation and physical properties of coral reefs. If the original coral reefs are directly used to prepare concrete, the requirements of aggregate size gradation in concrete cannot be satisfied. Therefore, the original coral reefs were crushed according to the size gradation of ordinary aggregates. Coral coarse aggregates (CA) with a diameter of 4.75–20 mm and coral sands (CS) with a diameter of 0.15–4.75 mm were obtained (as presented in Fig. 3). The particle size distribution of the coral aggregates are shown in Fig. 4. The physical properties of coral aggregates were tested according to GB/T 17431.2–2010 [23], as shown in Table 5. The results indicated that coral aggregates have high water absorption and porosity, and that the chloride ion content exceeded the standard limit. The bulk density of the coral aggregate is less than 1200 kg/m3, thus the coral aggregate is classified as light aggregates. The X-ray diffraction (XRD) results shown in Fig. 5 indicate that the chemical composition of coral aggregate is primarily CaCO3, which implies calcareous aggregate. Aragonite is the main mineral component of coral aggregate; as such, alkali aggregate reactions do not occur easily [1]. In addition, the microscopic characteristics of coral aggregate were observed by SEM. As shown in Fig. 6, many pores appeared in the aggregate endowing the aggregates with absorption and desorption abilities. 2.2. Mix proportions The details of all five mixtures used in this study are presented in Table 6. The fiber content is the ratio of the volume of the fiber to that of the concrete matrix. The proportions of BF in the five mixtures were 0%, 0.05%, 0.1%, 0.15%, and 0.2%, and these mixtures were denoted as BF0, BF5, BF10, BF15 and BF20, respectively. For all the mixtures, the water–binder ratio is 0.3 (excluding pre-wet water), and the concrete components except the fibers are the same. Coral aggregates with high porosity and water absorption

Table 1 Chemical composition of cementitious materials. Composition (wt. %)

OPC

FA

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Other

18.48 4.82 3.26 57.7 1.51 1.83 2.39

52.42 31.67 3.23 5.85 1.18 1.39 4.23

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D. Niu et al. / Construction and Building Materials 237 (2020) 117628 Table 2 Physical and mechanical properties of cement. Water content for standard consistency (%)

Specific area surface (m2/kg)

Soundness

Ignition loss (%)

25.8

334

Satisfied

2.79

Setting time (h)

Compressive strength (MPa)

Flexural strength (MPa)

Initial

Final

3d

28d

3d

28d

2.3

3.4

28.8

48.6

6.4

8.6

Table 3 Physical properties of fly ash. Water content (%)

Ignition loss (%)

Density (g/cm3)

Specific area surface (m2/kg)

28 d active index (%)

0.2

2.85

2.35

340

97

according to the water absorption of coral aggregate in 24 h and the working performance of BFRCAC under different pre-wetting rates. To obtain BFRCAC with uniformly dispersed fibers, the mixing time must be extended appropriately. The mixing procedure for preparing BFCRC is shown in Fig. 7. Before casting the specimens, a slump test of the mixtures was performed. The mixtures were cast into the prepared moulds and compacted on a vibrating table. Subsequently, the specimens were covered with a plastic film and demoulded after 24 h and then placed in a standard curing room of temperature of 20 ± 2 °C and relative humidity of exceeding 95% for 28 days.

2.3. Test method Fig. 1. Basalt fiber.

would significantly affect the workability of concrete. Therefore, it is necessary to pre-wet the aggregates in the preparation of concrete [24,25]. The amount of pre-wetting water is determined

2.3.1. Mechanical properties The compressive and splitting tensile strengths of all specimens (100 mm  100 mm  100 mm) were tested according to GB/T 50081–2002 [26]. The test ages were 3, 7, 28, 60, and 90 days. Three specimens of each mixture were tested in each group, and the average value was obtained as the final result.

Table 4 Physical and mechanical properties of BF. Type

Length (mm)

Diameter (lm)

Density (g/cm3)

Elastic modulus (MPa)

Tensile strength (MPa)

Elongation (%)

BF

18

15

2.56

75,000

4500

3.15

Fig. 2. Original coral reefs.

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D. Niu et al. / Construction and Building Materials 237 (2020) 117628

Fig. 3. (a) Coral coarse aggregates and (b) coral sands.

0

0 20

Cumulative sieve (%)

Cumulative sieve (%)

(b)

Coral coarse aggregate Upper limit areas Lower limit areas

(a)

40 60 80

100 2.36

4.75

9.6

16

19

Size (mm)

Coral sand II II

20 40 60 80

100 0.15

0.3

0.6

1.18

2.36

4.75

Size (mm)

Fig. 4. Particle size gradation of (a) coral coarse aggregate and (b) coral sand.

Table 5 Physical properties of coral coarse aggregate and coral sand. Material characteristics 3

Bulk density (kg/m ) Apparent density (kg/m3) Void content (%) 24 h Water absorption rate (mass %) Mud concentration (%) Cylinder compressive strength (MPa) Fineness Modulus (Mx) Chloride ion content (%)

Coral coarse aggregates

Coral sand

909 1724 47.3 15 1.0 2.27 – 0.025

1138 1538 23.2 16.7 1.5 – 2.8 0.05

1 CaCO3

1

2 SiO2 2

2

Fig. 6. SEM image of coral aggregate.

1 1 1 1

1

20

30

1

1

1

2 1

1

40

1

50

2 Theta (degree) Fig. 5. XRD pattern of coral aggregate.

60

2.3.2. Chloride content test The total and free chloride ion contents in BFRCAC were measured according to the acid soluble [27] and potentiometric [28] methods at 3, 7, 14, 21, 28, 60 and 90 days. A PXSJ–216F titrator with a PCl-1 chloride ion electrode and saturated potassium sulfate reference electrode was used to measure the free chloride ion in the filtrate. Samples were prepared according to the process shown in Fig. 8. Before testing, the sample powder was dried at 50 °C for 24 h in a vacuum oven to remove moisture. Subsequently, 3 g of

5

D. Niu et al. / Construction and Building Materials 237 (2020) 117628 Table 6 Mix proportions of concrete (kg/m3). Specimen

OPC

FA

CA

CS

W

Pre-wetting water

PBS

BF Volume fraction (%)

BF0 BF5 BF10 BF15 BF20

442 442 442 442 442

78 78 78 78 78

678 678 678 678 678

984 984 984 984 984

156 156 156 156 156

133 133 133 133 133

4.16 4.16 4.16 4.16 4.16

0.0 0.05 0.10 0.15 0.20

Fig. 7. Schematic diagram of the mixing procedure for BFCRC.

Fig. 8. Sample preparation procedure.

dry mortar powder was dissolved using 60 ml of distilled water and dilute nitric acid at a concentration of 1:10. The solution was shaken for 10 min on an oscillator and then placed for 24 h. Subsequently, the solution was filtered and the filtrate was used to measure the chloride ion content. The total and free chloride ions were calculated by Eqs. (1) and (2), respectively.

Ct ¼

Cf ¼

MðC Ag V  C KSCN V 1 Þ G VV 23 M  10

- pX

G

 V3

 100%

 100%

ð1Þ

ð2Þ

where Ct is the total chloride content, Cf the free chloride content, M the molar mass of chloride, CAg the standard concentration of AgNO3 solution, G the mass of mortar powder, V the consumed volume of AgNO3 solution, CKSCN the standard concentration of KSCN solution, V1 the consumed volume of KSCN solution, V2 the filtrate volume, V3 the soak volume. 2.3.3. Sorptivity Considering that ASTM C1585-13 [29] cannot be used to determine the evaporation loss of the specimens in the pretreatment process, it is difficult to ensure that the saturation and water loss of all the specimens to be tested are at the same level. Therefore, ASTM C1585-13 was improved according to Hall [30]. The specimen dimension was U100 mm  50 mm. The specimens were saturated for 24 h in vacuum and subsequently dried at 50 °C in a vacuum oven. The mass of the specimens was measured and the water loss was calculated daily. When the water loss reached approximately 50%, the specimens were sealed in a plastic bag and maintained in a standard curing room for 15 days to redistribute and balance the internal moisture.

When the specimens were removed from the curing room, the upper and lower surfaces were immediately pasted with an aluminum foil tape to prevent an internal water loss of the specimens. Simultaneously, epoxy resin was applied on one side of the specimen to ensure that the capillary absorption process of the concrete occurred as a one-dimensional absorption. After the resin had dried, the aluminum foil tapes on the upper and lower surfaces were removed, and the upper surface of the specimen was covered with a plastic film to prevent moisture evaporation from the upper surfaces during measurement. After the pretreatment, glass rods were placed at the bottom of the tank, the specimens were placed smoothly on the glass rods, and deionized water was slowly injected into the tank until the liquid level was (2 ± 1) mm higher than the bottom of the specimen. Subsequently, water penetrated gradually into concrete by capillary absorption. The mass of the specimen was measured at 0 min, 1 min, 5 min, 10 min, 20 min, 30 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 1 d, 2 d, 3 d, 4 d, 5 d, 6 d, 7 d, and 8 d. The water absorption and sorptivity of the specimen were calculated as follows:

i¼

mt aq

i S ¼ pffiffi t

ð3Þ

ð4Þ

where i is the cumulative water absorption at time t, mt the change in specimen mass at time t, a the exposed area of the specimen, q the density of water, t the capillary water absorption time, and S the sorptivity. 2.3.4. TG analysis TG analysis was performed on approximately 15 mg of a ground cement paste in open Al2O3 pans under N2 atmosphere at a heating

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D. Niu et al. / Construction and Building Materials 237 (2020) 117628

rate of 10 °C/min up to 900 °C. The bound water (BW) was computed between 50 and 550 °C from the TG data using Eq. (5).

BW ð%Þ ¼

M 50  M 550 M550

ð5Þ

must adsorb a large amount of free water and cement paste to wrap around, which renders the water film on the surfaces of the aggregate and cement particles thinner. Furthermore, the increase in the viscosity of the mixture results in a slump loss [31,32]. 3.2. Mechanical properties

3. Experimental results and discussion 3.1. Slump of concrete The effects of using different BF contents on the workability of concrete are shown in Fig. 9. As shown, the slump of concrete decreases as the BF content increases. The slump of the reference concrete is 185 mm. Adding BF at volume fractions of 0.05%, 0.1%, 0.15%, and 0.2% resulted in reducing the slumps to 130, 90, 75 and 59 mm, respectively. Thus, it can be concluded that the addition of fibers into CAC decreases the workability of the concrete. This is because the fibers can form a thick network skeleton in fresh concrete, which restrains the segregation of coral aggregates. Additionally, the gap between coarse aggregates decreases and the channel of fine aggregate flow is blocked. Owing to their larger specific surface area than that of the aggregates, the fibers

200 180 140 120 100 80 60 40 20 0

BF0

BF5

BF10

BF15

BF20

Fig. 9. Slump of CAC with various fiber contents.

Compressive strength (MPa)

Slump (mm)

160

60 55 50 45 40 35 30 25 20 15 10 5 0

(a)

BF0

BF5

3 days

7 days

28 days 90 days

60 days

BF10

BF15

BF20

The compressive strength and splitting tensile strength results of BFRCAC at 3, 7, 27, 60, and 90 days are shown in Fig. 10. Fig. 10(a) shows that the compressive strength at different ages can be improved by incorporating an appropriate amount of BF. The compressive strength of BF5 is the highest, which is 9.87% and 17.13% higher than those of the reference concrete (BF0) at the 28 and 90 days, respectively. However, the compressive strength decreases with the increase in fiber content, and the larger the fiber content, the more obvious is the strength decrease. BF20 shows the lowest compressive strength, which is 8.08% and 5.15% lower than those of the control concrete at 28 and 90 days, respectively. The same trend in strength change is described in literature [33]. These findings indicate that the optimum fiber content for the compressive strength of BFRCAC is 0.05%. When the fiber content is appropriate, the three-dimensional randomly distributed BF bundles restrict the transverse deformation of concrete under compression, and the fibers are closely bound in the matrix. When concrete is subjected to the load, stress expands along the aggregate interface and then transferred to the fiber position; additionally, the fiber bears part of the load according to fiber spacing theory. When the fiber is pulled out, the friction between the fiber and the concrete interface consumes a certain amount of energy, which consequently delays the destruction of the concrete. Hence, the compressive strength of concrete improves. However, when the fiber content is extremely high, the total surface area of fibers increases dramatically. Thus, more cement pastes are required to wrap the fibers, which affect the bonding effect between the cement pastes and aggregates. In addition, the excessive fiber content directly decreases the average spacing of the fibers in the matrix; when the average spacing is sufficiently small, crossover and overlap occur between fibers, causing the fibers to bond poorly with the paste to withstand external forces. Consequently, weak zones are formed that ultimately nullify the reinforcing effect of the fibers on the compressive strength, thus decreasing the compressive strength. Therefore, an accurate control of the fiber content of concrete is necessary [34]. As shown in Fig. 10(b), the improvement in the splitting tensile strength presents a trend similar to that of the compressive strength. BF has little effect on the early splitting tensile strength of concrete, but significant effect on the later splitting tensile

Splitting tensile strength (MPa)

2.3.5. FE-SEM analysis The specimen was crushed at 28 days. The resulting small fragments with BF were soaked in alcohol and dried in vacuum at 50 °C before testing. The microstructure was observed using the Zeiss GeminiSEM 500 Field Emission Scanning Electron Microscope.

5.5 5.0

3 days 28 days 90 days

(b)

4.5

7 days 60 days

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

BF0

BF5

BF10

BF15

Fig. 10. (a) The compressive strength and (b) splitting tensile strength of all mixtures at different ages.

BF20

7

D. Niu et al. / Construction and Building Materials 237 (2020) 117628

paste. At a later stage, with the hydration of the binder, the chloride ions diffuse gradually from the aggregates to the outside; therefore, the total chloride ion content increases with the curing age. After curing for more than 3 days, the concrete containing 0.05% fibers exhibit higher total chloride ion content than other concrete. The total chloride ion content in BF20 is lower than those of other specimens at 14–90 days. By adding 0.05%0.2% of BF into CAC, the total chloride ion content reduced by 0.48%5.8% at 3 days. At 90 days, the total chloride ion content of BF5 increased by 2.7%. However, the total chloride ion content decreased by 0.45%5% by adding 0.1%0.2% of BF. With the continuous hydration, the moisture in concrete decreases, and the pressure difference between the aggregate interior and exterior causes the prewetted water to be released. While the internal curing effect is achieved, chloride ions enter the concrete matrix from the aggregate interior with the water. The pore structure of BF5 is optimized, the internal curing effect is most apparent, and the chloride ion content is highest. The higher content of BF weakens the internal curing effect of concrete, which results in a lower total chloride ion content. As shown in Fig. 11(b), at the early stage of hydration, the free chloride ion content of BFRCAC decreases; however, at 14–28 days, the free chloride ion content increases. The free chloride ion content of all specimens is highest at 28 days. All the specimens containing BF present a higher free chloride ion content, which increases by 1.6%4.8% compared with that of the control concrete at 28 days. The free chloride ion of all specimens decrease rapidly at 60 days, which is 3.87%11.2% lower than that at 28 days. At 90 days, the hydration of BFRCAC is completed, and the free chloride ion content of all specimens is stable at approximately 0.12% 0.14%, which is below the critical chloride ion concentration of steel corrosion [35,36]. Some chloride ions carried by the coral aggregates are bound by cement hydration products during diffusion; thus, the free chloride ion content in concrete can be reduced. Two types of bound chloride ions exist: physically and chemically bound ions. For the former, chloride ions are adsorbed on the pore wall or hydration products by electrostatic or van der Waals force; the bound chloride ions is unstable and the ions can be released easily [37,38]. The chemical bond refers to the interaction between chloride and hydration products to form Friedel’s salt [39,40]. The bound chloride ion (Cb) can be calculated according to Eq. (6):

strength. At 28 days, the splitting tensile strength of concrete with 0.05% fiber content is the highest at 3.74 MPa, which is 1.36% higher than that of the control concrete. When the fiber content continues to increase, the splitting tensile strength decreases and becomes lower than that of the control concrete. The addition of 0.05%0.15% BF into concrete resulted in an increase of 4.41% 9.35% in the splitting tensile strength at 90 days. However, the splitting tensile strength decreases by 1.29% with the addition of 0.2% BF. Before cracking, the tensile stress of concrete is primarily borne by concrete. After cracking, the tensile stress begins to transfer from the matrix to the BFs between cracks. Through the redistribution of an internal force, the stress concentration factor at the micro-cracks of concrete is reduced, the ultimate tensile strain of concrete is increased, and the formation and expansion of cracks are prevented; thus, the splitting tensile strength is improved. However, when the fiber content is extremely high, the total surface area of the fiber increases and a large amount of cement paste is required. This negatively affects the bond between the cement paste and aggregate as well as the splitting tensile strength. Analysis of variance (ANOVA) values are listed in Tables 7 and 8. They indicate whether the compressive strength and splitting tensile strength difference between BFRCAC and the control concrete are significant. According to a defined significance level of 0.05, when the significance factor of BFRCAC is less than or equal to 0.05, a significant difference exists between its strength and those of the control samples. Otherwise, no significant difference is observed. Therefore, the addition of 0.05%0.2% BF would not significantly improve the compressive strength at 28 days compared with the control concrete. However, when the fiber content is either 0.05% or 0.1%, the compressive strength increases at 90 days, and a significant difference exists between BFRCAC and the control concrete. When the BF content is either 0.1% or 0.2%, the splitting tensile strength at 28 days decreases significantly, and a significant difference exists between BFRCAC and the control concrete. 3.3. Chloride content The total chloride ion and free chloride ion content with different ages of BFRCAC are shown in Fig. 11. Fig. 11(a) shows that the total chloride ion in BFRCAC increases with curing age. At the initial mixing stage, the chloride carried by coral aggregate dissolved in the solution of the ITZ between the aggregates and cement Table 7 Compressive strength values of all the specimens. Specimen

Compressive strength (MPa) 3 days

BF0 BF5 BF10 BF15 BF20

7 days

28 days

60 days

90 days

Ave.

Std Dev.

Ave.

Std Dev.

Ave.

Std Dev.

Sig.*

Ave.

Std Dev.

Ave.

Std Dev.

Sig.*

31.83 32.38 30.32 29.53 27.87

0.63 0.96 1.01 1.19 1.06

32.46 35.86 34.45 30.72 30.88

1.17 1.25 1.50 1.95 1.26

35.23 38.71 37.37 34.83 32.38

1.32 1.55 1.59 0.72 0.72

– 0.176 0.492 0.850 0.221

35.36 43.75 39.46 37.53 34.52

0.20 0.64 0.79 1.42 1.19

39.4 46.15 43.94 41.4 37.37

1.70 0.27 1.44 1.24 1.97

– 0.002 0.024 0.395 0.248

Table 8 Splitting tensile strength values of all the specimens. Specimen

Splitting tensile strength (MPa) 3 days

BF0 BF5 BF10 BF15 BF20

7 days

28 days

60 days

90 days

Ave.

Std Dev.

Ave.

Std Dev.

Ave.

Std Dev.

Sig.*

Ave.

Std Dev.

Ave.

Std Dev.

Sig.*

2.82 2.84 2.78 2.77 2.51

0.18 0.17 0.25 0.20 0.18

3.01 3.07 3.02 2.91 2.87

0.13 0.07 0.15 0.11 0.23

3.69 3.74 3.33 3.28 3.25

0.17 0.21 0.13 0.15 0.14

– 0.784 0.04 0.341 0.037

3.71 3.84 3.78 3.54 3.48

0.06 0.17 0.11 0.15 0.14

3.85 4.21 4.02 4.02 3.8

0.11 0.15 0.10 0.14 0.16

– 0.326 0.385 0.335 0.661

D. Niu et al. / Construction and Building Materials 237 (2020) 117628

Total chloride content ( % )

0.28

3 days 21 days 90 days

(a)

0.24

7 days 28 days

14 days 60 days

0.20 0.16 0.12 0.08 0.04 0.00

BF0

BF5

BF10

BF15

BF20

0.16

Free chloride ion content ( % )

8

(b)

0.14

3 days 21 days 90 days

7 days 28 days

14 days 60 days

0.12 0.10 0.08 0.06 0.04 0.02 0.00

BF0

BF5

BF10

BF15

BF20

Fig. 11. (a) The total chloride content and (b) free chloride content of all mixtures at different ages.

Cb ¼ Ct  Cf

ð6Þ

Fig. 12 presents the bound chloride ion content of BFRACA at different ages. The bound chloride ion content of the specimens increases with age except for the specimen with 0.05% fibers at 3–14 days. At 28 days, the bound chloride ion content decreases owing to the slowdown in hydration rate of the binder, and BF20 has the lowest bound chloride ion content. The addition of 0.05% BF into concrete results in the bound chloride ion content is highest at 90 days and the bound chloride ion content of BF20 is lowest. Adding the appropriate amount of BF in CAC can effectively control plastic shrinkage and reduce the formation of early microcracks, thus reducing the capillary of concrete, enhancing the internal curing effect in the later period, and producing more hydration products, resulting in the increase in bound chloride ion content at 60 and 90 days. However, the addition of excessive fibers increases the interface of concrete, which results in the increase in concrete porosity; therefore, water loss and drying shrinkage will become more apparent. Consequently, secondary hydration is weakened and the hydration products and bound chloride ion content decrease. 3.4. Sorptivity Water intrusion into building materials provides a path for the penetration of harmful substances such as chloride and sulfate ions. The main transport mechanisms of chloride and sulfate ions into concrete are diffusion and capillary action; however, diffusion

3 days 21 days 90 days

0.14

7 days 28 days

14 days 60 days

6 5

0.12 4

0.10

BF0 BF5 BF10 BF15 BF20

i (mm)

Bound chloride ion content (%)

0.16

alone is an extremely slow process. Therefore, capillary action may be the main transport mechanism, especially near the unsaturated concrete surface. Obviously, it is highly important to understand the moisture transport of concrete for evaluating its service life and improving its quality as a building material [41]. One of the methods to measure water entering concrete is through sorptivity test, which is used to determine the performance of near surface concrete. Surface concrete can control water ingress to concrete and steel. The water absorption or sorptivity of concrete can be regarded as a measure of capillary force produced by the pore structure, which causes water to be absorbed into concrete [30]. The sorptivity of concrete indirectly represents its porosity; furthermore, it provides useful information regarding the volume of pore permeability in concrete and the connectivity between pores. The cumulative amount of capillary water absorbed per unit area of BFRCAC at days calculated by Eq. (1) is shown in Fig. 13. Apparently, all specimens present an increase in water absorbed along with the measured times, but its growth rate decreases gradually. Martys et al. [41] concluded that as water is continuously inhaled, it encounters smaller gel pores, which are not easily invaded in water. Next, larger capillary pores exist in the interfacial zone around the aggregates, and stable or metastable meniscus forms at the interface between air and water, which hinders the continuous invasion of water, thus slowing sorptivity at the later stage. Adding the appropriate amount of BF into concrete can significantly reduce the water absorption of concrete; however, an excessive amount of BF will increase water absorption. The water

3

0.08 0.06

2

0.04

1

0.02 0

0.00

BF0

BF5

BF10

BF15

BF20

Fig. 12. Bound chloride content of all mixtures at different ages.

200

400

600

Time (s1/2)

Fig. 13. Capillary water absorption of BFRCAC.

800

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D. Niu et al. / Construction and Building Materials 237 (2020) 117628

Sorptivity (10-3mm/s1/2)

18

14 12 10 8 6 4

0

BF0

BF5

BF10

BF15

BF20

Fig. 14. Initial and secondary sorptivity of BFRCAC.

7.0 6.5

y=-0.28*x+15.64 R2=0.84

6.0 5.5 5.0 4.5 4.0

32

34

36

38

40

28-day compressive strength (MPa) Fig. 15. The relationship of sorptivity coefficients and 28 days compressive strength.

3.6. Porosity The theoretical total pore volume evolution in the samples was assessed from the TG data [42] according to Eq. (7).

Pð%Þ ¼

3.5. TG analysis

Initial Sorptivity Secondary Sorptivity

16

2

Sorptivity (10-3mm/s1/2)

absorption of the specimens containing 0%, 0.1%, and 0.15% BF is the same. The specimen BF5 exhibits the lowest water absorption, and BF20 the highest water absorption. By adding 0.05%, 0.1%, and 0.15% BF into CAC, the cumulative water absorption of concrete is reduced by 20.98%, 3.39% and 2.67%, respectively. Additionally, CAC containing 0.2% BF increases the cumulative water absorption by 18.83%. After an appropriate amount of BF is incorporated into concrete, a large number of uniformly distributed micro-bubbles is introduced, which refines the pore structure and reduces the pore connectivity. In addition, the three-dimensional disorderly distributed fiber system weakens the stress concentration of the microcrack end in the concrete and inhibits the extension of the crack to a certain extent, thereby improving the capillary water absorption resistance of CAC. Excessive fibers increase the bond interfaces in concrete, which have a small density and large voids, and easily connect with other voids through capillary pores to form a connecting pore. Second, when the fiber content is extremely high, the fibers are not uniformly dispersed; they overlap and agglomerate in the matrix, thereby resulting in the increase in coarse voids between fibers, which ultimately results in higher water absorption. The ANOVA results, as shown in Table 9, indicate that the addition of 0.05%0.15% BF decrease the cumulative water absorption, and 0.2% BF increase the cumulative water absorption. Based on the ANOVA, BF in CAC would not significantly affect water absorption compared with the reference concrete. The initial and secondary sorptivity were determined by the 6 h and 8 d cumulative water absorption, respectively. The obtained sorptivity coefficients are shown in Fig. 14. The initial and secondary sorptivity of all concretes exhibit the same trend. The sorptivity of BF5 is the lowest and that of BF20 is the highest. The specimen containing 0.05%0.15% BF reduced the initial and secondary sorptivity, which are 2.75%20.97% and 5.39%16.78% lower than that of the control concrete, respectively. A 0.2% of BF increases the initial and secondary sorptivity by 19.35% and 20.52%, respectively. It is concluded that the presence of appropriate BF in CAC plays a significant role in reducing water absorption and sorptivity. The relationship between sorptivity coefficient and 28-day compressive strength is plotted in Fig. 15, which reveals a good correlation. The sorptivity indicates a significant reduction with increasing compressive strength.

M w  BW Vp 1:3  100% ¼  100% Vw þ Vc M w þ Mq c

ð7Þ

c

TG data are used to analyze the content of the given hydration products in the specimens. The TG and differential thermogravimetric (DTG) curves of BFRCAC at 28 days of curing are shown in Fig. 16. The DTG curves exhibit endothermic peaks at 70–100 °C, 320–350 °C, 420–460 °C and 750–850 °C, which are related to the dehydration of adsorbed water or combined water of the hydration products (such as calcium silicate hydrate and calcium aluminate hydrate), the main Portlandite-like sheets of Friedel’s salt, Ca(OH)2 , and CaCO3, respectively.

Eq. (7) is used for cement mortar samples. Regarding concrete, Eq. (7) can be revised to Eqs. (8) and (9).

Pð%Þ ¼

M w  BW Vp 1:3  100% ¼  100% Vw þ Vs Mw þ Mq s

ð8Þ

s

Ms

qs

¼

Mc

qc

þ

MFA

qFA

þ

M ca

qca

þ

Mcs

ð9Þ

qcs

Table 9 Cumulative water absorption values of all the specimens. Specimen

BF0 BF5 BF10 BF15 BF20

6 h cumulative water absorption (mm)

8 d cumulative water absorption (mm)

Ave.

Std Dev.

Sig.*

Ave.

Std Dev.

Sig.*

1.94 1.54 1.87 1.89 2.31

0.015 0.167 0.001 0.006 0.011

– 0.309 0.541 0.664 0.084

4.65 3.87 4.35 4.41 5.55

0.013 0.536 0.176 0.009 0.204

– 0.275 0.443 0.161 0.111

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D. Niu et al. / Construction and Building Materials 237 (2020) 117628

100

(b)

(a)

°C

Weight loss (%)

95

0.1

90

0.0

Friedel's salt Friedel's salt

C-S-H

85

Ca(OH)2

-0.1

80 BF0 BF5 BF10 BF15 BF20

75 70 65 60

100

200

BF0 BF5 BF10 BF15 BF20

-0.2 -0.3 300

400

500

600

700

800

900

Temperature (°C)

-0.4

CaCO3

100 200 300 400 500 600 700 800 900

Fig. 16. (a) TG and (b) DTG curves of BFRCAC.

where P is theoretical total pore volume, Vw the mixing water volume, Vs the volume of other components (including OPC, FA, CA, and CS) without the mixing water volume, Mw the mixing water mass, BW the bound water mass, and 1.3 the average density of the chemically bound water [43]; Mc, MFA, Mca, and Mcs are the masses of C, FA, CA, and CS, respectively; qc, qFA, qca, and qcs are the densities of cement, FA, CA, and CS, respectively. This method provides neither the absolute pore volume of the pores nor the pore size distribution, but can be used as an assessment method [42,44]. The evolution of bound water and the porosity of BFRCAC are shown in Fig. 17. As shown, the bound water of the specimens increases and subsequently decreases, whereas the porosity decreases and subsequently increases with the increase in BF content. The porosity of BF5 is the lowest, which is 7.11% lower than that of the reference concrete. When BF of volume fraction 0.2% is added, the porosity is 2.26% higher than that of BF0. The addition of excessive fibers introduces defects and increases the theoretical total pore volume of concrete; defects essentially are loose matrix formed around fibers. This result is in agreement with the mechanical properties and durability of BFRCAC. 3.7. FE-SEM analysis To analyze the enhancement mechanism of BF in CAC, the microstructure of BFRCAC with different volume fractions of BF was measured through FE-SEM. Fig. 18 shows the FE-SEM micro-

8.7

9.2

8.6

4. Conclusions In this study, the mechanical properties, chloride ion content and capillary water absorption properties of BFCAC were evaluated. The conclusions are as follows.

9.0 8.8

8.3

8.6

8.2

8.4

8.1 8.2

8.0 7.9

8.0 BF0

BF5

BF10

BF15

BF20

Fig. 17. Porosity and bound water of BFRCAC.

Porosity (%)

Bound Water (%)

8.5 8.4

graphs of CAC with fiber volume fraction of 0.05%0.15%. As shown in Fig. 18(a), the BF are randomly distributed in the concrete matrix, forming a three-dimensional support network and bearing external loads in multiple directions, which can enhance the mechanical properties of concrete. Fig. 18(b), (c) and (d) show that the interface structures between the BF and matrix are compact, and that the bonding between the matrix and fiber is tight and strong. The BF is tightly wrapped in the cement paste and its bonding with cement paste is good. As shown in Fig. 18 (e), the hydration products cover the BF surface, which can form a large mechanical bite force with the cement matrix. The effect of BF dispersed in CAC is similar to that of reinforcing bars, and the secondary micro-reinforcing effect can effectively improve the mechanical properties and pore structure of the BFRCAC. The micrographs of the CAC with fiber volume fraction 0.2% are shown in Fig. 19. Fig. 19(a) shows the uneven dispersion of BF in CAC. Fig. 19(b) shows the interface between the BF and the matrix; the gap between the fiber and the matrix is clear. When BF of volume fraction 0.2% was added, the mechanical properties and durability of CAC deteriorated. This is primarily attributed to the inhomogeneous dispersion of the fibers, which resulted in the appearance of clusters of fibers. The separation between the fibers and the matrix can be attributed to the uneven dispersion and agglomeration of the fibers, and the insufficient adhesion between the fibers and the matrix [45]. Additionally, another reason may be the increasing water demand with increasing fiber content, which results in a decrease in the concrete compactness [46].

(1) Incorporation of BF into CAC contributed both positively and negatively to the mechanical properties of concrete. A 0.05% BF content has the highest improving effect on the mechanical properties, and the compressive strength and splitting tensile strength were increased by 9.87% and 1.36% at 28 days, respectively. However, when the fiber content continued to increase, the improving effect weakened or a negative effect was noted. Therefore, it is necessary to control the fiber content of concrete accurately. (2) The total chloride ion of BFRCAC increased with age. The free chloride ion content decreased with the curing age except for 21 and 28 days. The free chloride ion content in all spec-

D. Niu et al. / Construction and Building Materials 237 (2020) 117628

11

(b)

(a)

Basalt Fiber

(c)

(d)

(e) C-S-H

AFt

CH

Fig. 18. FE-SEM micrographs of fibers: (a) Distribution pattern of appropriate amount BF in BFRCAC and the interfaces between the BFs and matrix (b) 0.05% BF; (c) 0.1% BF; (d) 0.15%BF; (e) hydration products on the BF surface.

(a)

(b) Basalt fiber

Cracks

Fig. 19. FE-SEM micrographs of CAC with 0.2% BF (a) distribution pattern of BF and (b) interfaces between the BF and matrix.

imens was highest at 28 days. The dissolution of chloride ion was accelerated by adding 0.05% BF to CAC; however, the chloride ion dissolution was inhibited by adding 0.1%–0.2% BF.

(3) At the early stage of hydration, the bound chloride ion content of BFCAC increased with age. At 21 and 28 days, the bound chloride ion content decreased, and the bound content tended to be stable at 90 days. When the BF content

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D. Niu et al. / Construction and Building Materials 237 (2020) 117628

was 0.05%, the bound chloride ion content was the highest at 28 and 90 days, indicating that 0.05% BF could optimize the pore structure of concrete, and that the initial curing effect of coral aggregate promoted the formation of hydration products in the later stage. (4) The addition of BF to CAC decreased in water absorption and sorptivity of concrete significantly. The specimen containing 0.05% BF exhibited the lowest sorptivity. By increasing the fiber content, the improvement rate of capillary water absorption resistance decreased; when the BF content was 0.2%, an adverse impact on the water absorption was noted. The sorptivity and 28–day compressive strength exhibited a good correlation. (5) The inclusion of an appropriate amount of BF reduced the porosity of concrete, and the porosity of concrete increased when the BF content was extremely high. The appropriate amount of BF dispersed well in CAC and demonstrated reasonable bonding within the concrete matrix. However, the addition of excessive BF increased fiber agglomeration and debonding.

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